At one of our field offices, we have a field device that communicates over RS-485 (single two wire pair). Distance is approximately 50 yards through underground conduit.
We use a device which is essentially a RS-232 to RS-485 converter, that connects to the PC via RS-232. We have a tower in the office (Windows 7), that polls data from the device in the field. There are two lights on the RS-232 to RS-485 adapter device TD (transmit) and RD (receive).
This worked perfectly for several months, polling data every 10 seconds. The it suddenly stopped.
Connected to my laptop, it still works fine. When polling data, the TD blinks once, the RD blinks once, and the data shows up on the laptop.
When the device is connected to the tower, (same polling software, same 232/485 device drivers, same Windows OS), the TD light blinks once, and the RD light blinks constantly for 8 seconds (no readable data comes to the PC, looking at the raw data stream, the data coming back appears to be garbage).
So, I talked to the field device manufacturer and the converter manufacturer, no one has any idea, both say everything is fine if the laptop can connect.
So, I think it's got to be something with the PC. Maybe the serial port went bad or something like that. Today I try a second tower, this one with Windows XP, and it has the same problem. So, my laptop works, but neither tower works.
Could it be the power, some sort of grounding issue at the field office? My laptop is running off of battery power, and that's the only thing that I can think of.
Last thing I tried today, I took the tower to another field office, with the same device in the field, same adapter, plugged it in there, and it polled data perfectly. No change to the settings on the PC or anything, literally plugged it in and polling commenced within seconds.
Any suggestions, maybe a grounding issue or other thoughts?